Hydraulic suspension dampers typically comprise a tube filled with working liquid, inside of which a slidable piston is placed. The piston is attached to a piston rod led outside the damper through the piston rod guide, and has a form of a piston assembly comprising rebound and compression valves, which control the flow of working liquid passing through the piston during the rebound and the compression stroke of the damper. Twin-tube dampers also comprise a base (bottom) assembly with separate rebound and compression valves controlling the flow of working liquid passing in and out of the compensation chamber formed between the inner and the outer tube of the damper.
Each valve usually comprises a stack of resilient disks, covering the flow passages of the valve assembly, and acts as a one way valve deflecting or moving under the pressure of the working liquid so as to allow its flow. The valves are commonly preloaded with a compression spring usually supported on an additional seat abutting the disks stack of the rebound valve. At the other side the spring abuts a shoulder nut fixed to the piston or the base assembly. A number, shape, diameter, and thickness of each disk, as well as the preload pressure of the spring constitute the parameters used to adjust damping forces.
A damper of this kind is disclosed for example in the European publication EP 2233775 B1.
The shoulder nut is used to clamp all the components of the piston and/or base valve assembly together as well as to preload the compression spring, which by abutting the valve stack creates appropriate damping force dispersion. The assumed preload of the spring is therefore regulated by the assumed working height of the spring. This working height of the spring however does not depend solely on the designed distance between the shoulder nut and the stack of disks but is strongly influenced by the geometrical tolerances of all the elements supporting the spring as well as the spring itself. For example tolerances of the shoulder nut height within a range of +/−0.1 mm, and of the spring seat height within a range of +/−0.1 mm themselves may lead to variations in the spring working height In a range of +/−0.2 mm. Therefore operational characteristics inevitably vary to a large extent among dampers of the same production batch.